The present invention relates to a process of purifying oxygen-containing exhaust gases produced by the combustion of garbage, industrial waste materials and sewage sludge, wherein mercury, mercury compounds and polyhalogenated hydrocarbons are removed from the exhaust gases by adsorption on zeolites.
Because garbage, industrial waste materials, and sewage sludge have different and changing compositions, the exhaust gases produced by combustion of these waste materials are also contaminated with polluting substances in varying amounts. All impurities must be substantially completely removed from the combustion exhaust gases before it is permissible to discharge the exhaust gases into the atmosphere, since a very large number of impurities will, even in a low concentration, have toxic effects on human beings, animals and plants. The impurities contained in the exhaust gases produced by the combustion of waste materials include particularly dust, SO.sub.2, HCl, HF, Hg, mercury compounds, compounds of heavy metals, polyhalogenated dibenzodioxins and dibenzofurans, polychlorinated biphenyls (PCB), polychlorinated phenols (PCP) and polychlorinated aromatic compounds (PCA). Because the waste materials are burned with a hyperstoichiometric amount of oxygen, the combustion exhaust gases contain 2 to 12% by volume O.sub.2 per sm.sup.3 (sm.sup.3 --standard cubic meter).
The dust, which is contained in the exhaust gases in an amount of up to 50,000 mg/sm.sup.3, is separated in cyclones, electrostatic precipitators, cloth filters or scrubbers. The exhaust gas which is to be dedusted may optionally be conducted through more than one of such unit. The known dedusting processes, even when carried out on a commercial scale, permit residual gas contents below 5 mg/sm.sup.3. Dedusting must be effected to the highest possible degree because the dust mainly adsorbs toxic heavy metals, compounds of heavy metals and polyhalogenated dibenzodioxins and dibenzofurans.
SO.sub.2 and HCl are each contained in the exhaust gases in an amount of up to 3000 mg/sm.sup.3 and HF is contained therein in amounts of up to 100 mg/sm.sup.3. Together with the water vapor contained in the atmosphere the gaseous compounds form acids, which are very often present as aerosols, and the gaseous compounds have a toxic activity. For this reason they are substantially completely removed. The known purifying processes may be carried out on a commercial scale to effect a purification to residual contents of &lt;20 mg/sm.sup.3 SO.sub.2, &lt;5 mg/sm.sup.3 HCl, and &lt;1 mg/sm.sup.3 HF. SO.sub.2, HCl and HF are removed by dry or quasi-dry or wet purifying processes and it is possible to carry out a plurality of such processes in succession. The reactants used in these processes mainly include Ca(OH).sub.2, CaO, CaO.sub.3, NaOH, and/or Na.sub.2 CO.sub.3. A particularly high importance has been achieved by spray absorption, in which an aqueous suspension of Ca(OH).sub.2 reacts with the acid pollutants SO.sub.2, HCl, and HF, the water is evaporated, and a solid reaction product is obtained, which contains also dust and other pollutants. Scrubbing processes are also known, by which the pollutants SO.sub.2, HCl and HF can be removed to a very high degree.
The heavy metals, the compounds of heavy metals, particularly mercury and mercury compounds, and the polyhalogenated hydrocarbons are contained in the combustion exhaust gases in a lower concentration. But these substances have an extremely high toxic activity and for this reason must be almost completely removed from the exhaust gases. In the prior art this is preferably effected by adsorption processes and/or scrubbing processes. Adsorbents which have been found to be suitable include particularly activated carbon and zeolites. Scrubbing processes are carried out under acid or alkaline conditions.
The commercial use of the known processes of purifying exhaust gases particularly depends on whether they require only a low capital expenditure and low operating costs and results in process products which become available only at a very low rate and can be dumped without difficulty or can be recycled to the purifying processes after a regeneration. To separate the above-mentioned impurities to the highest possible degree, it is usual to combine a plurality of purifying processes. It is an object of the present invention to use zeolites as an adsorbent so that the use of activated carbon as an adsorbent and the resulting risks regarding the safety of the adsorption plants in operation can be avoided. Another object of the present invention is to provide an adsorption process which can conveniently be combined with the known processes for dedusting and for separating acid pollutants.
Published German Application 40 12 982 discloses a process of purifying gases and exhaust gases so as to remove inorganic and organic pollutants in which finely powdered zeolites and/or similar inorganic substances are blown into the gas stream, the gas and solids are thoroughly mixed, the solids are applied to a surface filter and are left on the filter so as to form a renewable loose adsorbent layer having a sufficient depth, and the pollutant-laden solids are mechanically removed from there. In the known process it is also contemplated that the inorganic substances having active surfaces are used as fine powders having a particle size distribution of 100% &lt;100 micrometers, preferably 100% &lt;50 micrometers. It is also contemplated in the known process that the inorganic substances having active surfaces are used as a finely powdered material having a particle size distribution of 100% &lt;100 micrometers, preferably 100% &lt;50 micrometers. It is also contemplated in the known process that the substances having active surfaces are contacted with sulfur, sulfides or sulfur-releasing substances to bind volatile heavy metals and/or contacted with heavy metals or compounds of heavy metals, such as vanadium, tungsten, molybdenum, manganese, iron, nickel, cobalt, chromium, copper, tin, zinc and/or titanium, in order to accelerate oxidation or reduction reactions. It has also been proposed in Published German Application 40 12 982 to use the known process for removal of mercury, cadmium, arsenic, antimony, thallium and/or of PCB'S, chlorinated hydrocarbons, dioxins/furans and/or polynuclear aromatic hydrocarbons.
Published German Application 39 19 124 discloses for the removal of noxious polycyclic hydrocarbons (such as dioxins and furans) and heavy metals from exhaust gases coming from waste incinerators a process in which the exhaust gases, after they have been prepurified to remove dust, HCl, HF, SO.sub.x, NO.sub.x and heavy metals are subjected to an adsorption/filtration at temperatures in the range from 70.degree. to 160.degree. C. to remove the remaining polycyclic hydrocarbons and heavy metals. In that process it is contemplated that the prepurified exhaust gases are first mixed in a reactor with finely divided adsorbents and are subsequently fed to a filtering separator, in which an adsorbent cake is formed on the filter cloth and is flown through by the exhaust gases. Polycyclic hydrocarbons and heavy metals are substantially completely removed in that known process, in which, e.g., activated carbon, molecular sieves, sodium sulfide, and hydrate of lime are used as adsorbents.
Finally, Published German Application 41 28 106 discloses a process for selective removal of highly condensed polycyclic hydrocarbons, particularly of halogenated dibenzodioxins and dibenzofurans, from prededusted exhaust gases which contain SO.sub.2, H.sub.2 O and heavy metals, in which the hydrocarbons are adsorbed on a solid adsorbent consisting of a de-aluminated zeolite having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 20:1 to 1000 to 1. The adsorption is effected at a temperature of from 20.degree. to 200.degree. C. The zeolite has a particle diameter of 1 to 5 mm and is disposed in a reactor in a fixed bed or moving bed.
It has been found that the known purifying processes must be improved because more stringent requirements regarding the purifying efficiency must be complied with.